Signaling with phase reversals



H. NYQUIST SIGNALING WITH PHASE REVERSALS Nov. 3, 1925.

Filed Aug. 28, 1923 4 Sheets-Shbet 3 T0 limo/"jig Transnulssiwa IN VENTOR l/jywls't iii/pa 'ATTORNEY Nov. 3 1 I v H NYQUIST SIGNALING WITHPHASE REVERSALS FiledAug. 1923 Film J1 orning Darwm t.

IN VE TOR Z fiQTTORNEY 4 Sheets-Sheet Patented Nov. 3, 1925.

HARRY NTYQUIST, OFiELMHURST, NEW YORK, ASSIGNOR. TO AMERICAN TELEPHONEAND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK. v

SIGNALING WITH PHASE BEVE'BSALS.

Application filed August 28, 1923. Serial 1T0. 659,754..

To all whom it may concern:

Be it known that I, HARRY NYQUIST, residin at Elmhurst, in the county ofQueens and tate of New York, have invented certain Improvements inSignaling with Phase the same circuit, the channels being so arrangedthat by the transmission of an alternating current of given magnitude, asigdi nal response of greater amplitude will be produced. Another objectof the invention is to provide a circuit of the type above described, inwhich the efiect of foreign interference will be reduced. Another objectof the invention is to provide a multiplex telegraph system so arran edthat the mutual interference between I will be a minimum. A stillfurther. object of the invention is to provide a tele' aph system inwhich the biasing efiec't of t e .received 'si als caused by variationsin the transmisslon equivalent of the circuit will 80 be racticallyeliminated.

hese objects, together with other objects of the invention, areaccomplished by transmitting an alternating current of constantfrequency and amplitudeand by reversmg the phase of the transmittedcurrent in response to the marking and spacing signals. At the receivingstation, the transmitted current will be combined with another cur rentof equal amplitude and of the same f uency but with its phase constant.While, in general, this controlling current may be either generated atthe transmitting station and transmitted to the receiving station, orgenerated at the receiving station under the control of the currentsource at the transmitting station, the specific embodiment of theinvention herein disclosed illustrates only the former of these methods.The i'nventlon may now be more fully understood from the followingdetailed description thereon. when read n connection with theaccompanying drawlng, Figure 1 of which illustrates a number of curvesshowing the form of wave transmitted in 'flerent channels accordancewith the present invention; 'Fig. 2 of which shows a schematic circuitvdiagram embodying the transmitting apparatus of the present invention;Figs. 3 and 4 of which illustrate two different types of recelvingarrangements to be employed in connection with the present invention;Figs. 5 and 6 of which illustrate two different cir cuit arrangementsfor producing phase reversals; Fig. 7 of which illustrates a piece ofapfparatus adapted to reduce the effect of lnter erence at thereceivingstation; and Fig. 8 of which illustrates curves showing the o eration ofthe apparatus of Fig. 7

Re erring to Fig. 1, the curve designated a illustrates the currentcondition in the line of an alternating current telegraph system spacingsignal corresponds to the no-current condition of the line, theparticular signal illustrated being the letter I. Curve 6 represents thesame letter when current in I the line corresponds to the markingcondition. The curve designated 0 represents the same letter formed inaccordance with the present invention by sim ly reversin the phasewithout altering t e magnitu e of the current. It will be seen that if asignaling wave such as is illustrated by the curve a is combined at thereceiving end with the simple sine wave of the same frequency andconstant amplitude as shown b the curve marked 1, the resultant wave wi1 be identical either with curve a or b, depending upon whether theconstant sine wave be made to correspond in phase to the markingcondition or the spacing condition of the si al wave. Inasmuch as thereceived signa is the result of two waves 0 and d, the signal willhavean amplitude twice as great as the signal produced by waves such asshown' A signal wave such as that illu'strated at 0 will be advantageous(provided a wavesimilar to d can be supplied at the re-' at a or b.

ceiving end) either from the standpoint of producing a stronger signalat the receiving station, or from the point of view of obtaining asstrong a signal as can be at resent attained, but with a line currentonly alf as great'as that required for signaling methods of the priorart.

Fig. 2 illustrates the apparatus necessary at a transmitting stationwhere the present invention is applied to a multiplex carrier telegraphsystem. For purposes of illustration, two signaling linesL and L areprovided, each having four channels transmitting thereover at differentcarrier frequencies. The number of channels transmitting over each linemay, of course. be made either greater or less in number than thatillustrated. The carrier currents for the four lines may be supplied,for example, from four sources of 500, 700, 900 and 1100 -cycles,respectively, and the several sources may supply as many signaling linesas are necessary to carry the desired trafiic. Gommon to each of thesignaling lines is a control line L the purpose of the latter being totransmit the control frequencies corresponding to each of the channelsofeach signal line. Where, for example, four carrier channels areprovided for each signal line, there will be four different signalingfrequencies transmitted over each of the lines and four constantamplitude, constant phase and constant frequency currents will betransmitted over the control circuit L But one control circuit will benecessary, regardless of the number of signaling lines in-, volved inthe system, and consequently, the provision of the control circuit will,in general, represent but a small additional expense over and above thatof the transmis sion lines themselves when the important advantages inoperation are taken into consideration.

Each of the carrier frequency sources, as, for example, the 500 cyclesource, is connected to common bus-bars as illustrated at 10, and fromthe bus-bars connections lead through suitable filters or otherselecting devices F F and F, to the control circuit L and the varioustransmitting lines such as L and L. Each of the other sources will beconnected; to the control circuit L. and the various transmission linessuch as L and L'- through similar groups of filters as illustrated. Aphase correcting or controlling device of well-known character isillustrated at 14 in connection with the control circuit for the purposeof adjusting the phase of the control frequency to the desired relationwith the signaling current, as will be described later, one of thesephase controllers being provided in connection with each carrier source.

. For the purpose of simplicity of illustration, the signaling mechanismfor producing the current reversals in the signaling frequencies appliedto the circuits L and L are shown as being simple pole changers such as18, 18, etc. Simple pole changers of this type are not, however, welladapted .for producing the phase reversals in practice for the reasonthat a certain amount of time must elapse for the armature of the polechanger to pass from one contact Accordingly, it is preferred that somearrangement such as is illustrated in Fig. 5 be utilized for producingthe phase reversals. Fig. 5 illustrates a bridge arrangement comprisingthree arms, each of resistance R, and a fourth arm including the contactof a relay 22, whereby when the contact is closed, the resistance of thefourth arm will be zero, and when it is opened, the resistance will beinfinite. Circuit connections extend from two opposite corners of thebridge to the carrier supply source (not illustrated) and circuitconnections from the other two corners of the bridge extend throughsuit-able apparatus (not illustrated in Fig. 5) to the signaling line.Preferably, the impedance looking towards the source and looking towardsthe signaling line should have the same value R as the arms of thebridge. It will be obvious fromthis circuit arran ement that if the keyK controlling the cmcuit of the relay 22 is closed, so that the fourtharm of the bridge is of zero resistance, the bridge will be unbalancedto transmit current of a definite amplitude to the line, while if thekey is opened, so that the the opposite sense, thereby causing a currentof equal amplitude to flow in the opposite direction. In the case of analternating current supply, it will be obvious that this will produce achange in phase of 180 in current supplied to the line. Since thischange in the phase. is instantaneous upon the making or breaking of thecontact, there will be no hiatus between reversals.

A modified arrangement is illustrated in Fig. 6. In this figure, abalancing arrangement comprising a hybrid coil 23 and a balancingresistance or network 24 and a circuit 25 is provided. The currentsupply may heconnected, for example, to the midpoints of the windings ofthe hybrid coil, the circuit to be supplied being in turn connected tothe secondary of the hybrid coil, although the circuit will work equallywell if the current source is connected to the secondary and the circuitto be supplied be connected to the midpoints of the hybrid coil. It theimpedenoe of the circuit 25 be made just equal to that of the network24, it will be apparent that no current will be. transmit-tedfrom thesource to the circuit to be supplied. lhe circuit 25, however, containsthe contact of a relay 22 and when the contact of the relay is closed,the circuit 25 has zero resistance, while with the contact open, thecircuit 25 has infinite resistance. In the one case, the circuit will beunbalanced in one direction, and in the other case, the circuit will beunbalanced in the opposite direction, so that by manipulating e key K toopen and close the contact of the relay 22, reversals in phase of thealternating current supplied will occur, and these reversals will occurinstantly upon the making or breaking of thecircuit.

Itwill be noted in connection withjthe circuit'of Fig. 2 thatinasmuch asthe controlled currents transmitted over the line L5 come from the samesource as the signaling currents transmitted over the lines L, L, etc.,they will be of the'same frequenc as the signaling currents themselves.Furt ermore, inasmuch as the controlled currents pass over a circuitsubstantiallythe same as the signaling circuits, they wlll be subject tothe same phase changes and hence suitable for combination in proper"phase relation with the signaling currents received at the receivingstation. In connection with the phase relation, it will be noticed thatthe phase relation of the arriving currents at the receivingstation may.be adjusted by means of the phase correction device connected in serieswith each source supplying the control circuit, and once the phase isfixed for each frequency, it will remain properly adjusted withoutfurther attention. It will further be observed that apart from theaddition of the control circuit, the arrangement shown at thetransmitting station involves substantially the same units of apparatusas are provided for ordinary multlplex carrier telegraph systems exceptthat the transmitter conslsts of a reversingarrangement instead of theusual device for opening and shorting the circuit. The

wavesof each frequency resulting from the operationof the transmittingkey will correspond in form to the wave shown at c in fii 3 illustratesone arrangement whereby t e signals may be received. The signaling linessuch as L and L terminate at the-receiving station in hybrid coils suchas 26 and 26', the opposite terminals of the hybrid coils bein connectedto common bus bars leading to t e incoming control circuit ,L It is, ofcourse, desirable thatthe control currents supplied to the terminals ofthe hybrid coil be equal in amplitude to the signaling currents suppliedto the opposite terminals. Since, however, the current transmitted overthe, control circuit L, supplies a plurality of receiving circuits andwill, in general, be substantiall no greater in amplitude duringtransmisslon over the line than the current transmitted over eachsignaling circuit, it is necessary to su ply amplifying arrangementssuch as A an A between the terminals of each hybrid coil and thebus-bars of the control circuit. By

a suitable adjustment of the amplifiers (which may of any type wellknown in the art), the control current sup lied to the hybrid coil maybe made equa in amplitude to the signaling current.

The secondary winding ofeach hybrid I coil is connected to bus-bars suchas .27 and 27 to which the individual receiving channels are connected.-The signaling channels corresponding vto the signaling line L includefilters such as RF,, RF RF, and RF each selective of the particularcarrier frequency assigned to the channel, each receiving channel alsoincluding a detector such as 1),, D etc. Thedetectors may be of anywell-known type, such as ordinaryvacuum tube detectors, and as isusuallythe practice in carrier telegraph circuits, the receiving elementassociated with the detector comorises a polar relay havingone windingin the plate circuit and another winding in an auxiliary circuitincluding a resistance properly proportioned with respect to the plateresistance of the tube, the latter circuit being also supplicdwithcurrent from the plate source. The circuitof the detectin tube is soadjusted that when no signal is eing received, nocurrent flows through ithe upper winding and the current in the lower winding produces a pulltending to shift the armature of the relay in one direction with a givenforce. When a normal signaling current is being received, however,

sufiicient current flows in the plate circuit to energize the upperwinding of-the polar relay to produce a force substantially twice asgreat, tending to shift the armature in the opposite direction.- Thisresults in a properly unbiased polar 'rela action for a received signalof given amp itude.

Returning to the hybrid-coil 26, it will be observed that balancingresistances or networks 28 and 28', are'associated with the hybridcoils. portioned with respect to the impedance 901G112 into thereceiving channels that the circuits leading to the signaling line andthe control circuit are con ugate with respect to each other. By meansof this arrangement, the incoming signal wave of the form shown in curve0 of Fig. 1 is su erposed upon the control wave of the form s own by thecurved to produce in the secondary circuit of the hybrid coil a wave ofeither the type illustrated by curve a or the type illustrated bycurve6, depending upon the phase relation of the control wave to the signalwave. At the same time, a complementary wave will .be produced in thenetwork 28 or 28, as the case maybe. That is to say, if the wave in thesecondary assumes the form of curve a, the wave in the network willassume the form of curve, b, and vice versa.

This may be readily understood by con.- sidering that if the wave 0,during the marking inter-val, is transmitted through the lhese networksare so profrom the signal.

lower windings of the hybrid coil to the terminals of the network inopposite phase relation to the wave at transmitted through the upperwindings of the coil, a wave such as I), having twice the amplitude ofthe signal wave, will be produced in the network during the markingperiod but no wave will be induced in the secondary of the hybrid coilbecause the waves oppose each other. During the spacing period, thesignal wave and the control wave will be series-aiding in the hybridcoil and will consequently produce a wave of double amplitude in thereceiving circuit. No current will, however, be transmitted to thenetwork during the spacing period. On the other hand, if the phase ofthe control wave be changed 180, the opposite effect will be produced,no current being transmitted to the network during the marking and adouble marking current being transmitted to the receiving terminal.During the spacing period, no current will be transmitted to thereceiving terminal but a double current willbe transmitted to thenetwork. In short, the operationgof the hybridhcoil arrangement is'suchthat the signaling wave such as 0, transmitted 'over a signal line, willbe transformed at the receiving terminals into a wave either of type aor of type b, having twice the amplitude of the transmitted signalingwave. As the received wave is of the ordinary type, it is consequentlydetected after being selected into the proper receiving channel by adetector of ,the ordinary type. Consequently, the only additionalfeature required by the receiving circuit over and above the apparatusused in ordinary carrier telegraph systems is the hybrid coil fortranslating the signal current from a wave of type 0 to a wave of type aor b, as the case may be.

It will be observed that the signals received by means of thearrangements shown in Fig. 3 vary in amplitude as the transmissionequivalent of the circuit varies. This introduces a bias into thereceived si nals due to the fact that the detected signal ing currentflowing through the upper winding of the polar relay will notnecessarily be twice as great as'normal current flowing in the lowerwinding. Since it is not, in general, practicable to vary the biasingcurrent in the lower winding to correspond to the change in theamplitude of the detected signaling current, it follows that the arran ement of Fig. 3 from the standpoint of has is no better than an ordinarycarrier tele graph circuit of the usual type. The operation in thisrespect, however, may be greatly improved by supplying the biasingcurrent v For example, it will be obvious that if marking currents aretransmitted through the filters illustrated to the detectors to operatethe upper winding of the polar relay, the spacing currents which aretransmitted to the network might be selected similar filters andtransmitted through similar detector tubes to energize the lower windingof the polar relay, thereby avoiding any biasing effect due to change inthe transmission equivalent. As this is not a very practicable methodowing to the duplication of filters and detectors, it is preferred touse an arrangement such as shown in Fig. 4 for accomplishing the sameresult.

In accordance with Fig. 4, duplex balanced vacuum tube detectors areemployed instead of single vacuum tube detectors such as shown in 'Fig.3. The signaling current having the form of wave 0 is led throughfilters such as RF RF etc., in the various receiving channels to theinput circuits of the detectors as indicated, in such a manner as toproduce opposite potentials upon the grids of the tubes. The severalcontrol frequencies are selected by receiving filters such as RF, RFetc., and transmitted over a circuit inductively connected to the commonconductor of the input circuits of each duplex detector. As aconsequenceof this connection, the'control wave potential will be addedto the signal wave potential upon the one grid, While the two waves willoppose each other upon. the other grid. Depending upon the phase of thecontrol wave, it is possible by a proper adjustment of the detecting tues to impress only marking currents upon the upper tube, for example,and only spacing currents upon the lower tube. The detected component ofthe marking current will then energize the upper winding of the polarrelay to shift the armature in the one direction, while the detectedcomponent of the spacing current will energize the lower winding toshift the armature in the opposite direction. No biasing efi'ect willtake place with this arrangement, because the current in each winding ofthe polar relay depends to the same extent upon the transmissionequivalent of the signaling and control circuits.

While the methods of signaling above discussed result in doubling theamplitude of the received signal, it should be noted that with thesystem disclosed we will have foreign interference from two sources,namelythe foreign interference which is introduced through the signalingcircuit, and also the foreign interference which is introduced in,

the control circuit. While at first thought it might se'e'mthat thesetwo interfering currents would tend to combine in the same manner as thesignal currents to produce a greater interference in the receiver, itwill be found upon closer analysis that such is not the case. Owinto-the fact that the signaling and contro currents are controllable inphase, the phase relations of these two currents will be fixed toproduce the maximum amplitude of the signal. The

two interfering components obtained from the control and signalingcircuits, respectively, continually Vary in phase with respect to eachother and will, on the averagg, tend to neutralize each other in art.For example, if We consider two interfering Waves received in thecontrol circuit and a signaling circuit, it will be apparent that therelative phases of the two components will vary from zero to 180 andthat, on

interference which is introduced in the control circuit. The arrangementconsists of a vacuum tube distorting device 40 having a high rsistance41 in its input circuit and a sharply tuned circuit or filter 42 in itsoutput circuit. If we consider a simple sine wave applied to the inputcircuit of the vacuum tube, the effect is to cut off the top and bottomof the wave, thereby tending to roduce in the output circuit a more oress square-topped wave. (See curve Y of Fig; 8.) The square-topped wave,as is well-understood, is equivalent to a wave of the original frequencyhaving superposed thereon various harmonics of the original frequency.The tuned circuit or filter 42 will then have the effect of selectingthe fundamental frequency and eliminating the harmonics, so that thewave finally transmitted will be substantially a pure sine wave. Supposenow the sine wave. impressed upon the input circuit is subjected tointerference of such a nature as to var the form of the wave to thatillustrated by the curve X of Fig. 8. Obviously, any interference suchasthat shown at n, o and p, which tends to increase or decrease theamplitude of the wave, will be to alarge extent eliminated by passing itthrough the distortin tube, since the interference will be out 0 whenthe to and bottom of the wave are cut off by t e distorting circuit.Interference such as that shown at m, tending to slightly widen thewave, will, however, result in a slight widening of the' square-toppedwave so that in this particular pulse there will be a slight distortionas indicated. Even this distortion, howlever, to a large extentdisappears upon passing the square-top ed wave through the filter andsubstantial a pure sine wave will be transmitted as indicated by thecurve Z.

In the arrangement of Fig. 4, it is, of

course, necessary to provide in each channel leading from the controlcircuit an amplifier such as A A A etc., for the purposeof making thecontrolcurrent equal in amplitude to the received signaling current.While this amplifier may be a simple vacuum tube amplifier of theordinary type, it is preferred to modify the circuit as illustrated .inFig, 7 to produce a distorting effect upon the applied wave at the sametime that it is amplified, thereby to a large extent eliminating theinterference components from the control wave and producing as theeffective control wave a wave such as is shown by thecurve Z of Fig. 8.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departin from thespirit of the invention as definein the following claims.

What is claimed is:

1. The method of signaling, which consists in transmitting a waveofconstant amplitude and frequency but reversed in phase at intervals toindicate signals, and combining at the receiving station with the wavethus transmitted, a wave of substantially the same amplitude andfrequency but unreversed in phase.

2. The method of signalin which consists in transmitting a wave 0%constant amplitude and frequency but reversed in phase at intervals toindicate signals, transmitting to the receiving station a similar waveexcept that it is unreversed in phase, and combining the two waves atthe receiving station.

3. The method of signaling, which consists in generating a wave ofconstant amplitude and frequency, applying a portion of the energy ofsaid wave to each of two circuits, reversing the phase of the wave applied'to one circuit at intervals, and combiny mg the wave reversed inphase with'the unreversed wave at the receiving station.

4. The method of signaling, which consists in generating a wave ofconstant amplitude and frequency, applying a ortion of v the energy ofsaid wave to each 0 a plurality of circuits, transmitting the wave overone of said circuits without phase reversals, producing phase reversalscorresponding to signals in the components of said wave transmitted overothers of said circuits, and separately combining the unreversed wave atthe receiving station with each of the waves transmitted over the othercircuits to produce signals corresponding to each circuit.

5. In a signalin system, means for transmitting a wave 0 constantamplitude and frequency, means for producing phase reversals in saidwave in accordance with signals, means at the receiving station tocomquencybut unreversed in phase.

6. n a signaling system, means for trans- 'mitting a Wave of constantamplitude and frequency, means to produce phase reversals in said wavein accordance with signals, means to transmit to the receiving station awave of substantially the same amplitude and frequency but unreversed inphase, and means to combine the two waves at the receiving station.

7. In a signaling system, means for generating a wave of constantamplitudeand frequency, means to apply a portion of the energy of saidwave to each of two circuits, means to reverse the phase of the waveapplied to one circuit in accordance with signals, and means at thereceiving station to combine the wave reversed in phase with theunreversed wave.

'8. In a signaling system, a plurality of signaling circuits and acontrol circuit, means for generating a wave of constant amplitude andfrequency, means to apply a portion of the energy of said wave to eachof the signaling circuits, meansto produce phase reversals in the waveapplied to each signaling circuit, means to apply a portion of theenergy of said wave to said control circuit, and means at the receivingstation to separately combine the wave transmitted over said controlcircuit with the waves transmitted over the signaling circuits.

9. In'a signaling system, a plurality of signaling circuits and acontrol circuit, means to generate a plurality of constant amplitudeWaves of diflerent frequencies, means to apply a portion of the ener ofeach of said waves to said control clrcuit without phase reversals,means to apply a portion of the energy of all,of said waves to each ofsaid signaling circuits, means associated with each signaling circuitfor producingsphase reversals in each of the waves applied theretoinaccordance with separate signals, means at a receiving station toselect from each signaling circuit the individual waves with their phasereversals, and means to combine with each selected wave of each circuita corresponding wave received from the control circuit which isunreversed in phase.

10. In a signaling system, means to transmit to a receiving station awave of constant amplitude and frequency reversed in phase at intervalsin accordance with signals, means to transmit to said receiving stationa wave of similar amplitude and frequency but unreversed in' phase,means to separate interference components from the latter wavecomprising a distorting device for rendering the wave substantiallysquare-topped, selective means for selecting a wave of sine form fromthe distorted wave, and means to combine the selected wave with the wavewhich is reversed in phase to produce a signal.

11. In a signaling system, a transmitting device comprising a balancedtype of circuit, a single source of current supply for said circuit anda contact in said circuit so arranged that when the contact is openedthe .circuit will be unbalanced in one sense with respect to said sourceand when the contact is closed the circuit will be unbalanced in the 0posite sense with respect to said source.

12. n a signaling system, a transmitting arrangement comprlsing a.single source of current, a circuit to which current from said will betransmitted to said first mentioned source in opposite phase relation.

13. In a signaling system, a transmitting arrangement comprising asource of alternat' ing current, a circuit-to be supplied with currentfrom said source, a balanced type of circuit associated with said firstmentioned circuit, connections from said source to neutral points ofsaid balanced type of circuit, and a. contact in said balanced type ofcircuit so arranged that when the. contact is opened the circuit will beunbalanced in one direction andwhen it is closed it will be unbalancedin the opposite direction.

14. In a signaling system, means to transmit an alternating signalingcurrent of, constant amplitude and frequency. in which marking andspacing intervals are differentiated by reversals in the phase of thecurrent, a detecting device comprising .two detecting elements, meansto-actuate one element with currents of one phase and means to actuatethe other element with currents of the opposite phase.

15. In a signaling system, means to transmit an alternating signalingcurrent of constant amplitude and frequency in which marking and spacingintervals arefldifl'erentiated by reversals in the phase of the current,a detecting arrangement-comprising a pair of detecting elementsga polarrelay having a winding in circuit with eachv detecting element, means totransmit a detected current through one winding of said relay inresponse to currents of one phase, and means to transmit a detectedcurrent through the other winding of said relay in response to currentsof the opposite phase.

16. In a signaling system, means to transmit a signaling current ofconstant amplitude and frequency in which marking and spacing intervalsare differentiated by reversals in the phase of the current, means tocombine said current/with a current of substantially the same amplitudeand frequency but Without phase reversals to produce resultant currentcomponents, one of which has constant amplitude current during a markinginterval and no current during the spacing interval and the other ofwhich has no current during the marking interval and constant amplitudecurrent during the spac- -ing interval a pair of detecting devices, 10

means to impress one of said components upon one detecting device andmeans to impress the other component upon the other detecting device.

In testimony whereof, I have signed my 1 name to this specification this27th day of August, 1923. y

- HARRY NYQUIST.

